Since You Asked
This page will be used to publish, from time to time, answers to
questions we receive.
Miscellaneous things you might be wondering
Readings - Appendix A and online references as indicated in the notes
January 28
Your Profile
Have you taken 1901?
| Yes |
92 |
| No |
4 |
| Didn't answer |
2 |
Do you know Java?
| Yes |
18 |
| No |
60 |
| Some/Minimal |
18 |
| Didn't answer |
2 |
Succeeding in this course
- Tutoring?
- "I have no absolutely no prior knowledge of Java, I'm scared"
- Don't be... most of your fellow students don't either. You can
learn the Java you need through paying attention in class,
working in the lab, doing the readings, and working on the
homework assignments. Come to see any of the course staff if
you're having trouble.
- How do I study successfully for this class?
- Come to class. Engage in the activities.
- Do the readings
- PROGRAM: Doing the lab and homework assignments is a minimum. The
book is a good source of questions and exercises which you can use
as self-tests.
- Work through the review material I prepare for each quiz. Be sure
you understand the topics and can do all the sample problems.
Programming assignments
- How do I find partners for the homework assignments?
- Talk to people in your lab session and in class. It's absolutely
fine to have a partner who's in a different lab session.
- How long will the programming assignments take? (if I want to get an
A).
- It's hard to answer precisely: I'd guess 5-10 hours for the first
assignment and 10-20 hours for the rest.
- What aren't we doing any "big projects"?
- Big projects: high reward, high risk... For the same reason, we
have four quizzes (and you can drop the lowest score) instead of
two mid-terms
Quizzes/exams
- When is the final?
- Good question: I'll have an answer by next week.
- Where can I find practice problems?
- I will post sample problems/questions for each quiz. The book is a
good source, too.
- Are there unannounced quizzes?
- Are exams "open book open notes"?
- All the exams are closed-book; however, you may bring one
letter-size piece of paper, two-sided, of handwritten notes.
- What is the format of quizzes?
- I will post lots of sample questions. However, in general, you
will have to write some code fragments and answer written
questions to show your understanding of concepts
- Studying for quizzes
- I will post topics that will be covered and sample questions
- I will go over in class briefly, ask for your questions
- There may be outside class review sessions, but there is no
guarantee of this
Labs
- Must finish by the end of class - they're designed that way
- Not graded. Just complete the (required) activity for credit ("check
mark")
- What if you didn't get a check mark in the first lab?
Grading
- "Will there be a curve?"
- Extra credit?
- Often the assignments have a little extra credit. Sometimes the
quizzes do, too.
Content questions
Note: Many of these will be covered in detail later in the semester
- How do you measure efficiency of algorithms?
- Find the right units to count (like "comparisons")
- Count how "long" (in terms of these units) the program will take to
run in the worst case
- Express "how long" as a function of the "size of the input"
Questions about Java
- This class uses only Java, not other programming languages
It covers only the fundamental aspects of Java. Java is a huge
language (with many class libraries), and we will not cover most of
them. Things we will not cover include:
- applets, web programming
- graphical user interfaces
- What are advantages of Java?
- write once, run anywhere
- industry standard language
- security model: Web applets
- networked
- object-oriented: a better way to build software
- huge set of libraries
- Is Java used for more than just internet-based programs?
- History of Java?
- How long do programming languages last?
- Some last a long time:
- Fortran, Cobol, Lisp, ...
- Why are we using Java 5, not Java 6?
- Java 5 introduced several key features to the core language that
are relevant and useful to the type of program you'll write in
this course, notably generics and autoboxing. (I will cover these
topics later). You may use Java 6, but your programs must compile
in Java 5, since that's what we're using.
- How different would this class be if you were using another language
or programming environment?
- It would probably be about 20-30% different. You'd learn that
language instead of Java, obviously. The core of the class would
stay the same, however: you'd still learn the same data
structures, algorithms, and analytic techniques.
- Many questions about Java specifics that will be answered over the
next few weeks
- What is "object-oriented programming"?
Eclipse
Eclipse is open source, freely available. Works for all
platforms. You can download it to your computer (if you have one) and
do all your work there. However, you must ensure that your program
runs on ITLabs machines and create a file to submit. See the
instructions for Assignment 1 and the FAQ.
February 13
Java
Abstract Classes
- Why would you want to declare abstract methods in an abstract
superclass?
- They are a sort of "contract" or even "documentation in
code". That is, at the level of the abstract class, one can say
that this behavior exists, but cannot implement it. However, any
subclass of the abstract class must implement the abstract method
(or, if it does not, the subclass will be abstract too).
- What are abstract classes used for?
- Where "there is common functionality that you'd like to implement
in a superclass, and (b) some behavior is unique to specific
classes and cannot be factored into the superclass." Case (a)
gives rise to methods that you actually implement in the abstract
class. Case (b) gives rise to abstract methods.
- Must an abstract class have a subclass?
- Yes. Since you can't make an instance of an abstract class, if you
don't define subclasses of the class, it will be of no use.
The equals() method
When we define the equals method for our class, are we overriding or
overloading?
CLARIFICATION: The example I gave last time was "wrong"... I was
overloading, not overriding as I intended. The reason is that the
parameter of equals must be of type Object in order to override the
method inherited from object. Technically, the signature must be
exactly the same as the signature of the inherited method.
The tricky thing is that this problem may manifest itself right away:
if you invoke equals directly, the results will be as you
expect. However, the equals method is called automatically by lots of
methods that are used frequently for Lists, such as indexOf and
contains. Theese methods (indexOf, contains, etc.) call the equals
method with the signature:
public boolean (Object o)
so if you wrote an equals method whose parameter is not of type
Object, it will not be invoked in these cases. Instead, the default
equals method from the Object class will be used. THIS WILL NOT BE
WHAT YOU WANT.
Overriding vs. Overloading
"I still don't understand overriding vs. overloading" clearly.
Can you make a class private?
No, at least not "top level" classes (i.e., classes that are not
defined within another class). There are only two options for
a top level class:
- public
- package-private. If you DO NOT specify the public access modifier
for a class, this is the default. This means that only classes
within the same package as this class can access it.
Soon we'll see that you can define a class within another class.
These so-called "inner classes" may be private or protected. They can
be public, too, but often you don't want them to be public.
How much of Java will we learn?
- How "in depth" will we go into Java?
- Will we learn anything about creating applets?
- Are we going to learn anything about the newest version of Java? If
not, can you point out the major differences?
We are learning only enough to work through the data structures and
algorithms that are the focus of the course. We've learned most of
what we will need. Things we're still going to cover (most of them
right now):
- Interfaces
- Generics
- Enums
- autoboxing
- Iterators
- Exceptions
- Logging
- Some built-in interfaces and classes, such as List, ArrayList,
LinkedList.
We will not cover applets. We are not going to use features of the
latest version of Java (6) because they are not necessary for the type
of code we'll be writing. However, you should be aware that we will
make heavy use of features introduced in Java 5 (generics, enums,
autoboxing) because they are extremely useful for the type of code
we'll be writing.
I still don't understand the difference between private and public clearly
See the text (p. 10) or
http://java.sun.com/docs/books/tutorial/java/javaOO/accesscontrol.html
"Any classes can use a public method, but a private method can be
used only by the class that defines it."
"Still not clear about 'this'"
See pages 12, 22, and 50 in the text
Within the body of a method "this" refers to the object that invoked
the method. Therefore, this.someVariable is a way to refer to the
someVariable instance variable of the object a method was invoked
on. And this.someMethod(...) is a way to invoke another method on the
object.
You can invoke one constructor from another constructor. That's where
the this(...) syntax comes in. See p. 22-23 in the text.
Enumeration / enums
See pp. 26-27, and 872-873 in the text.
What are all the Java keywords?
Look on the inside front cover of the textbook.
Why is the Object class ever used?
It isn't used much anymore (in part due to the introduction of
generics in Java 5). One place you're still likely to use it is in
equals methods that you write.
Structure of a large Java application
"does everything need to be contained with a larger class?"
No. Public classes (and their public methods) can be used from
"anywhere".
What does the "new" operator do?
It creates an instance of the specified class, for example:
Customer c = new Customer("Fred", 12345);
creates a new Customer object.
How similiar are Java and C++?
Hard to answer simply. From "far enough away", extremely similar. Once
you get closer, there are significant differences in the details. But
if you know Java, learning C++ will be pretty easy. If you want to get
an overview of the similarities and differences, see
http://en.wikipedia.org/wiki/Comparison_of_Java_and_C%2B%2B. Or do a
google search "Comparing Java and C++". You'll see lots of web
articles that do just that.
Grading, Assignments, and Homework
Assignment 1
You'll get your grades and feedback today
Labs
- What happens if you don't get 10 lab check marks?
You don't pass the class. However, THIS HAS NEVER HAPPENED! So if
you are falling behind in the labs, please see me at once!
- If you don't finish the lab on time, please talk to your TA to see
what you can do to receive credit
- Can you use classes/methods in your homework that we haven't gone
over in class?
It depends. Often you're instructed specifically to use certain
classes and methods because that's a big part of the assignment.
If that isn't the case, however, typically you can use whatever
classes/methods you want. Please ask me if you're not sure.
- Can test cases be posted earlier?
We'll always post some right away, but we want to give you time to
think through the assignment on your own. Think of the test data
and test cases that are posted a day before the assignment is due
as a "rescue" opportunity helping you notice any incorrect
assumptions you made in your solution before it's too late.
General Requests, Suggestions, and Questions
- "I wish we would go over more actual writing code"
We will do so... I'll give you more exercises to write code, and
I'll go over more and more code in Eclipse during class
- Use more diagrams when explaining
More diagrams will be forthcoming.
- "Can you post the Java code you go over in class?"
Yes, I always do. Typically the lecture notes and code are posted
by the next day
- Please post the lab Monday night, not Tuesday at 9 am
If it's ready by then, we will
- Will we be concentrating more on data structures or algorithms?
Both pretty equally
- How can I get a private tutor?
- "When will the pace of the class pick up?"
Now!
- "Will we catch back up to where we're supposed to be?"
Probably not, but we will not get any further behind.
- "If I have a hard time following in class, does the book provide
enough info to understand everything necessary for the course?"
- Probably yes
- BUT, if you don't follow in class, I'd urge you to come talk to
me in office hours, so I can try to help understand why you
don't, and enable you to do so.
- Resources for studying other than the textbook
- What's the difference between object-oriented and functional
programming?
A good source: The Wikipedia articles on each topic
February 27
Java
If you want to really understand Lists and other "collections" in
Java, you should check out
[http://java.sun.com/docs/books/tutorial/collections/index.html].
This will also help you understand the use of generic types better
See also: http://java.sun.com/docs/books/tutorial/collections/interfaces/list.html
Comparing the pros and cons of ArrayList and LinkedList
Since Java already has provided impelementations of the List
interface, when would it be useful to develop your own implementation?
- In most cases, you will not do so. The major reason that we do it in
class is so you can understand how it's done.
What's the difference between the length of an array and the size of
a List?
- Conceptually, the difference is that the length of an array tells
you how many memory locations have been allocated for that
array. Put another way, it tells you the MAXIMUM number of elements
the array could hold. The size of a list tells you how many
elements actually are in the list.
How can you 'swap' the positions of 2 items in an ArrayList?
temp = al.get(i);
al.set(i, al.get(j));
al.set(j, temp);
Note that the same logic would be used for an array
Generic types
- "The E[] syntax"???
- See pp. 43-45 and much of chapters 4 and 5 in the text
- See also http://java.sun.com/j2se/1.5.0/docs/guide/language/generics.html
- Why should you declare a variable to be of type List<E> rather than
ArrayList<E>?
- Whenever possible, avoid implementation commitments: for
example, if you write a method that takes a List as a parameter,
then it will work whether the List is implemented as an
ArrayList, LinkedList, or Vector (or other possibilities).
- Will you learn about input?
- How to write data to a file?
Access modifiers: private vs. public
- See pages 10-12 of the text
- When should you use private methods?
- For the same reason as you declare private instance variables:
when the method is conceptually "internal" to a class. Earlier,
we had only discussed methods that needed to be public because
they constituted the external "application programmer interface"
for a class, i.e., the functionality that a class wanted to make
available to the outside world (other classes). However, as we
saw when implementing a List, sometimes it is convenient for a
class to define methods that are only called by other methods
within the class. For an array-based implementation of a List,
for example, the doubleArray() method doesn't need to be
externally visible. Even more strongly, since it would "give
away" something about the internal implementation of the list,
one would not want to make it public.
- What "kind of thing" is an array?
- See the extensive index entry on arrays in the textbook. Start
with pp. 890-897.
- The use of type casting in the .equals method
- See the text, pp. 59-61
- The parameter of the .equals method is of type Object. However,
you really want to check the equality of two objects of a more
specific type of data (Customer or Card or Album
or...). Typecasting is a way of telling Java, "Look, this object
that (so far) you only think of as an Object actually is a
Card!".
Enums
- How do you increment an enum?
- You don't increment an enum, exactly. However, the values()
method will give you an array of all the values in an enum. You
then can iterate through the array (typically using a for-each
loop).
- How do you use the parameters of objects in enums?
- See the getShape and getColor methods in Suit.java (Assignment 2)
Can you rename a class after it's been created?
- Not while your program is running. Of course, you can always change
the name of any object in your code, as long as you change it
consistently.
Implementing PI (or other irrational numbers) in Java
How do you refer to objects you create?
- They must either be stored on a variable (i.e., that you can access
by name) or be stored as part of a data structure (e.g., an array
or List) that is stored on a variable.
How to use random number generators
- See the documentation for the class Random() or the method
Math.random()
Programming Issues
- How important is it for your programs to be efficient?
- For the purposes of the assignment, time efficiency typically is
not an issue (unless that's part of the point of the assignment).
However:
- A key tool for a software engineer is understanding
efficiency, so he or she can select the most efficient (and,
more generally the most appropriate) algorithm for a given
problem.
- Further, "good design" — e.g., following Java conventions,
structuring modules that make sense and are of a reasonable
size, using interfaces and abstract classes appropriately,
etc. etc. — is crucial. We will note this — and grade
accordingly — whenever we have the opportunity.
- Is our array-based implementation of the List interface efficient?
- Yes. In its essentials, it's quite similar to java's ArrayList
implementation.
- "Why should you every use arrays? Lists seem better".
- You're right: Lists usually are better. However, there are
circumstances when you want to use arrays and reasons for
knowing how to use arrays, which I believe I've addressed in
class. To summarize:
- If efficiency really matters and/or the collection of data
you're storing is of a predictable and relatively fixed size,
use arrays.
- Second, we take two perspectives on abstract data types in
this class: user and implementer. When you use Java's List
interfaces and supplied classes (e.g., ArrayList), you're
taking the user perspective. However, it also is important
for you to know how to define ADTs: so far, we've done that
with our array-based implementations of List.
- Why should we use exceptions? Why not just print out an error
message?
Course Issues
Pace of class
- We're now going about as fast as we should be. If the pace feels too
fast for you:
- Read the book, paying attention to the questions and problems
- Form a study group
- Come to office hours
Will there be extra credit?
- Yes, typically the homework assignments will have extra credit. The
quizzes and final may, but there is no set pattern here
Can I post sample problems online to solve?
- I will try, but I can't guarantee it.
- Am I available outside my office hours?
- Send me email, and I will try to find time. However, I cannot
make any guarantees that I will be able to do so.
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